Each capacitor in a switched capacitor circuit can be described as acting either as a capacitor or as a resistor. A capacitor in a switched capacitor circuit is said to act as a capacitor when the capacitor has memory of a previous value, and passes current that is proportional to dv/dt (i.e., I (current)=Cdv/dt). Such a capacitor generally performs some type of frequency shaping because the capacitor passes more and more current as the frequency of the voltage increases across the terminals of the capacitor. In contrast, a capacitor in a switched capacitor circuit is said to act as a resistor when the capacitor has no memory of a previous value, and passes current that is proportional to a voltage (i.e., V=iR (voltage=current*resistance), or equivalently I=CV/T (wherein T is the period of a sampling clock)).
Unlike with a capacitor, the value of the current passed by a true resistor is independent of the frequency of the input signal. FIG. 1 illustrates a circuit 20 wherein the capacitor 22 therein is said to act as a capacitor--i.e., the capacitor passes more current as the frequency of the input signal, V.sub.in, increases. Hence, the current (i) provided to the capacitor is equal to C*dv/dt. In contrast, FIG. 2 illustrates a circuit 30 wherein the capacitor 32 therein is said to act as a resistor--i.e., the capacitor passes a current proportional to the magnitude of the input signal, V.sub.in. Hence, the current (i) provided to the capacitor is equal to CV/T. In FIG. 2, the waveforms of two non-overlapping clock signals, clk1 and clk2, are depicted under the circuit 30. While clock signal clk1 controls switch 1 in the circuit, clock signal clk2 controls switch 2.
It is generally advantageous in discrete time frequency shaping circuits to use circuits that have minimal attenuation and provide feedthrough isolation between samples (wherein v.sub.out is not directly coupled to v.sub.in). FIG. 3 illustrates a typical continuous-time analogous circuit 40 wherein the circuit 40 includes a feedthrough isolation sample and hold portion 42 in front. A corresponding discrete-time analogous circuit 50 is illustrated in FIG. 4, and is provided by replacing each resistor shown in FIG. 3 with a capacitor and providing switches that move i=CV/T. In FIG. 4, the capacitor C.sub.shape still acts as a capacitor, and passes current that is proportional to dv/dt (i.e., i (current)=Cdv/dt).
FIG. 4 also illustrates the structure of the sample and hold portion of the circuit. As shown, while the resistive-type capacitors, C.sub.g and C.sub.f, in the zero portion already have direct feedthrough isolation, capacitor C.sub.shape does not have direct feedthrough isolation without the sample and hold circuit in front. Adding a switch in front of capacitor C.sub.shape (instead of providing the sample and hold portion) will not provide direct feedthrough isolation because when the switch is closed, v.sub.in will directly change v.sub.out. FIG. 4 illustrates the traditional method of implementing the required sample and hold function. As shown, two active stages are implemented (the sample and hold portion and the zero portion), and each requires a large area, power-consuming operational amplifier. However, it is advantageous to limit the area of a circuit as well as limit the power consumed by a circuit. For example, with regard to Ethernet chips, as the number of required ports (i.e., channels) continue to increase, package power dissipation is becoming more and more of a critical issue. Cutting power consumption, such as in half, for a stage in a circuit is significant because more ports (i.e., channels) can be added to the same chip while maintaining safe die temperatures.